Electromagnetic actuators are well known. In many applications, the output force of the actuator is controlled by and is a function of an electrical control or command signal, and as such can be used in a variety of applications.
For example, as described in U.S. Pat. No. 5,187,398, issued Feb. 16, 1993 and assigned to the assignee of the invention disclosed in the present application (hereinafter the "'398 Patent"), one type of a linear electromagnetic actuator is described. The electromagnetic actuator of the '398 patent includes a magnetic flux conductive material case, an electrical current conductive coil, a magnetic flux conductive material core, and a pair of radially polarized magnetic flux developing elements. The coil is disposed in the case coextensively adjacent to its interior wall. The core is moveably received within the chamber with motion of the core occurring between the first end and the second end of the case such that a first region of the core traverses the coil between its first end and its midpoint, and a second region of the core traverses the coil between its second end and its midpoint. A first one of the magnetic elements is carried by the first region and a second one of the magnetic elements is carried by the second region so that magnetic flux across the coil between the first region and the case is in a first direction, and magnetic flux across the coil between the second region and the case is in a second direction. The coil is arranged so that an electrical current in the coil between the first coil and the midpoint flows in an opposite direction with respect to the direction of the current in the coil between the second coil and the midpoint. Therefore, the flux current cross product of the flux in the first direction with the coil current and the flux current cross product of the flux in the second direction with the coil current are additive.
Many known electromagnetic actuators use permanent magnets. Permanent magnet actuators however have several disadvantages and limitations. For example permanent magnets are fragile and are therefore unsuitable for use in actuators for rugged applications. Permanent magnets also experience a loss of polarization at high temperatures. Therefore permanent magnet actuators operating at high temperatures may experience a reduction in the flux density through the gap, thereby reducing efficiency of the actuator. Permanent magnets are also expensive, adding to the overall cost of the permanent magnet actuator.